Posts Tagged ‘electric generator’

Last time we learned how enthalpy is used to measure heat energy contained in the steam inside a power plant. The higher the steam pressure, the higher the enthalpy, and vice versa, and we touched upon the concept of work, or the potential for a useful outcome of a process. Today we’ll see how to get the maximum work out of a steam turbine by attaching a condenser at the point of its exhaust and making the most of the vacuum that exists within its condenser.

Let’s revisit the equation introduced last time, which allows us to determine the amount of useful work output:

W = h1 – h2

Applied to a power plant’s water-to-steam cycle, enthalpy h1 is solely dependent on the pressure and temperature of steam entering the turbine from the boiler and superheater, as contained within the purple dashed line in the diagram below.

As for enthalpy h2, it’s solely dependent on the pressure and temperature of steam within the condenser portion of the water-to-steam cycle, as shown by the blue dashed circle of the diagram.

Next week we’ll see how the condenser, and more specifically the vacuum inside of it, sets the platform for increased energy production, a/k/a work.

Last time we added a piece of equipment called a superheater, positioned between the boiler and steam turbine,to our basic electric utility power plant steam and water cycle. Its addition enables a greater and more consistent supply of heat energy to the steam which powers the turbine. How much more? Let’s look at Figure 1 to get an idea.

Figure 1

You may have noticed that our illustration lacks numerical representation. That’s because power plants are designed differently, depending on fuels used and power output required. So unless we’re talking about a particular power plant, number values would be impractical. For example, I could specify a boiling point of 596°F at 1,500 pounds per square inch (PSI), and a superheater outlet temperature of 1,050°F at 1,200PSI, and I could make note of esoteric things like enthalpy (British Thermal Units per pound mass) values on the Heat Energy axis. But to facilitate our discussion we’ll keep things simple and focus on the general process.

Figure 1 shows in phase D the additional heat energy being added to the steam, thanks to the superheater. This is significantly more than had been added by the boiler alone, as represented by phase C. The turbine consumes heat energy added in phases C and D and converts it into mechanical energy to drive the generator, resulting in electrical energy being provided to consumers in the most energy efficient way possible.

But increasing power output and efficiency isn’t the superheater’s only job. The heat it adds during phase D ensures the turbine’s safe operation when it’s cranking at full capacity, as represented by the superheated steam zones of phases C and D.

Next week we’ll discover how the superheater prevents a destructive process known as condensing from occurring inside the turbine.